support `cargo test`

It would be nice to better support for the `cargo test` subcommand. We should look into getting json data from rustc which describes the test results. I'm not sure if this data is available in json format, in which case, it might be worth hacking together a parser which reads the stdout output from `cargo test` and parses it into a json structure.

Then we could have a nice ui component for displaying successful/failed test cases.

Further ideas welcome!

Start by investigating how cargo test exposes test-result data through rustc or stdout, as described in the issue. Review the existing cargo command handling and UI entry points, if present, then define the supported result structure and test-case display before implementation; done means successful and failed test cases are represented in the browser interface.

Written by the indexing model from the issue text.